1. Field of the Invention
This invention relates to an inkjet printing system for printing an image on a print medium using an image improving liquid in addition to an ink. Particularly the present invention relates to an inkjet printing system and an inkjet printing method for, in a case of using an ink containing a pigment in a colorant, reducing coloring of reflected light due to thin film interference or bronze of an image while securing image clarity.
2. Description of the Related Art
In a recent inkjet printing market, there is provided an inkjet printing apparatus where there is a demand for output of an image with a high grade comparable to a silver photograph and also weather resistance of the outputted image, and a pigment ink having high robustness of a colorant itself is used. In the image printed by the pigment ink, however, for example, a new harmful effect to an image such as a phenomenon (for example, bronze) of reflecting light having a color different from a colorant on a surface of the image is confirmed. Hereinafter, the above harmful effect to the image will be briefly explained.
In general, the phenomenon of reflecting the light having the color different from the colorant on the surface of the image is brought in by thin film interference or bronze. The thin film interference is a phenomenon which occurs in a case where a thickness of a printed colorant layer is in sync with a wavelength of light and in which a color of the reflected light changes depending on a reflection angle, that is, an observation angle. A printed matter distinguishing in coloring of the reflected light is observed with a color different from a color desired to be originally expressed by an observer, giving a discomfort feeling. On the other hand, the bronze is considered as a phenomenon occurring as a result that when pigment colorant particles are exposed on a surface of a print medium, a ratio of wavelength components in an absorption band of the pigment increases in the reflected light by selective reflection of light on a pigment particle surface. Distinguishableness of the bronze differs depending on the kind or an amount of the colorant, for example, in a case of using cyan pigments, the reddish reflected light is visible.
For reducing the coloring of the reflected light to be generated due to such thin film interference or bronze, there is proposed a method for laminating a transparent film on a print surface to prevent pigment particles from being exposed on the surface of the print medium. In addition, there is proposed a method for putting an additive such as titanium dioxides to a colored ink.
Further, Japanese Patent Publication No. 4066338 discloses a technology of over-coating a print medium with a yellow ink. This is a method in which, after forming an image on the print medium by using cyan, magenta, and yellow inks, the image is over-coated with the yellow ink causing less bronze in a low print ratio, thus reducing the bronze in a cyan hue particularly.
In addition, there is proposed a method in which a non-colored, transparent clear ink (image improving liquid) is prepared in addition to the pigment ink for image formation and is then applied on an image, thus preventing exposure of the pigment and restricting occurrence of the thin film interference to reduce the coloring of the reflected light.
In the method for laminating the transparent film, however, there occur various problems such as an increase in cost of the apparatus due to a provision of the laminate mechanism or an increase in hours or labors required in the laminate operation. The method for putting the additive such as titanium oxides into the colored ink raises a problem of ejection instability or the like. Further, since in the specification of Japanese Patent Publication No. 4066338, not the clear ink but the yellow ink is used, an entire image including a blank region is yellowish, therefore narrowing a color expression region or losing a gray balance.
Further, also in a method for coating a pigment surface with a clear ink, there is a possibility that glossy properties, particularly image clarity of a print medium is damaged. For example, in a case where a print is performed on a glossy paper by a pigment ink, since the pigment ink tends to be easily left on the surface, convexity and concavity are formed on the surface of the print medium. When the clear ink for bronze prevention is applied on the print medium surface in such a state, layers of printing portions are more highly laminated to develop the convexity and concavity to be larger on the surface. As a result, the coloring of the reflected light due to the bronze or the thin film interference is certainly alleviated, but the image clarity is remarkably reduced to bring in a new image problem. Therefore, in a case of restricting the coloring of the reflected light by the clear ink, the characteristic and the application amount of the clear ink are required to be adjusted in consideration of the coloring of the reflected light and the image clarity in such a manner that both of them can be restricted within a degree of being not problematic on the image.
On the other hand, according to the study of the present inventors, the appropriate application amount of the clear ink depends on the feature and the application of the image to be printed. Specially as comparing a case of printing a color photograph using many kinds of pigment inks (CMYK) with a case of printing a monochromatic photograph using one or two kinds of pigment inks (K and Gy), the degree of the coloring of the reflected light originally differs therebetween. On top of that, the deterioration degree of the image clarity also differs at the time of applying the same amount of the clear ink. In addition, a range or accuracy in color reproduction to be required also differs between the color photograph and the monochromatic photograph. That is, between the color photograph and the monochromatic photograph, the characteristic and the amount of the clear ink to be applied will differ with each other for restricting both the coloring of the reflected light and the image clarity to be within a range of being not problematic on the image.
Particularly in recent years, for meeting a demand for a color photograph realizing “a wide color reproduction region” or a monochromatic photograph having “an excellent gray balance”, there is provided an inkjet printing apparatus for preparing particular printing modes (color mode and monochromatic mode) for realizing these color and monochromatic photographs. In the meantime, image output having no problem on the coloring of the reflected light and the image clarity is expected in any of the printing modes. In any of the conventional methods, however, the kind and amount of the clear ink can not be thus adjusted based upon the feature or the application of the image, and it is difficult to output the image in which the coloring of the reflected light and the image clarity are restricted within an allowance range in every mode.